Photopolymerizable compositions featuring novel amine accelerator for improved color stability and reduced polymerization stress thereby

ABSTRACT

A photopolymerization accelerator composition improving color stability and controlling polymerization shrinkage stress of cured resin and/or the resulting composite paste thereby feature by tertiary twisted biphenyldiamine with the general formula I: 
                         
R2 and R3 are each independently alkyl having from 1 to 5 carbon atoms; R and R1 are each independently hydrogen or halogen; alkyl, alkoxy, or alkylthio having from 1 to 18 carbon atoms; or phenyl and/or substituted phenyl alkoxy, or alkylthio having from 1 to 18 carbon atoms. It can be used in part with conventional photosensitizers and radically polymerizable monomers.

BACKGROUND OF THE INVENTION

Light curable composition finds many applications in paints, coating,optical and microelectronic adhesives, dental adhesive and composites etal. UV and visible light are two common light sources to promote suchcuring reaction. Furthermore such a light curing process can also beclassified as photo-induced cationic polymerization or photo-inducedfree radical polymerization based on the nature of photo-initiator andmechanism of polymerization. For any given light curable compositionthere is at least composed of a curable resin and photo-initiator.Typical curable resin include (meth)acrylate, vinyl, vinylether, epoxy,et al. Photo-initiators are photo-sensitizer, such as arylketone,diketone, acylphosphine oxide et al. Besides additional coinitiator oraccelerator is also required in order to produce an effective curing. Ithas been found that tertiary aromatic amines, such asEthyl-4-DimethylAmine Benzoate (EDAB), are the most effectiveaccelerators. However, the accelerating efficiency between thesetertiary aromatic amines depends heavily upon the substitution on thearomatic moiety. Moreover there is increasing concerns over thepotential toxicity and the long-term effect on color stability for theresulting cured systems based on those conventional tertiary aromaticamines. It was believed color stability was influenced more by thenature of the aromatic ring than by the substituent on the nitrogenatom. Discoloration of the cured matrix containing such a tertiaryaromatic amine is the increasing concern, especially when it is used asdental restoratives where color match is much more critical. Thereforethere is a need for readily obtained, reactive, and color stable amineas novel photo coinitiator.

For dental application, it is more desirable to use photoinitiator whichis color stable but demonstrate high ambient light (environmental light)stable and promote quick polymerization later on. Such a photoinitiatorsystem composing an α-diketone, an aliphatic/aromatic amine and atriazane derivative, was disclosed in US Paten Application 2004/0180983.However, it is concerned about its potential problems in color stabilityand shrinkage stress concentration associated with for such a initiatorsystem due to the nature of the isomerization nature of the triazinecompound and the fast cure reaction rate.

One reason behind color shifting of cured dental resin and composite isattributed to the residual amine, which is generally a tertiary aromaticamine. UV or thermal aging can cause its isomerization to form coloredstructure. One solution to disrupt possible conjugation through suchstructural isomerization of aromatic moiety by making a substitutearomatic and isolated such aromatic ring. 2,2′, 6,6′-tetrasubstitutebiphenyl have been explored to make colorless polymers.

Another issue associated with free radical-based light curing process isthe shrinkage and shrinkage stress due to the fast curing nature ofconventional free radical photopolymerization. Therefore there is needto develop new photo-initiator which can demonstrate good balancebetween quicker curing rate and lower curing stress for a novel freeradical curing system. It is the intention for this invention is tofurther tune down the radical polymerization rate in an effort to reducethe shrinkage stress for a given cured composition.

It is well known that the origin of stress in adhesive resin compositerestorations is due to the restrained shrinkage during polymerizationand it is dependent on the configuration of the restoration. Moreover,non-homogeneous deformations during functional loading can damage theinterface as well as the coherence of the material. Damage from thesestresses can be reduced by application of elastic lining at the adhesiveinterfaces and by slowing the initial conversion by two-step lightinitiation of the resin. The various factors that mediate flow andcompliance are all have something to do with polymerization stressbuildup or failure of a restored tooth. In addition to the nature ofresin composition, how a given resin or composite is cured is alsocritical to the total stress development, which means a kinetic controlon the polymerization stress development is possible. With increasingMW, polymer chain mobility was limited, the diffusion become the ratecontrol factor. In a comparison with linear system, the limited mobilityin a cross-linking system appear to come earlier, which means extrareaction would lead to an increasing polymerization stress. Althoughsuch a cross-linking reaction could not allowed scarifying to exchange alow stress because it did contribute the mechanical property to thefinal material.

There are different approaches to control the stress generation anddevelopment:

-   1. Slow down the polymerization rate;    -   Introducing a special rate controller like stable radicals or        P&P resin system developed recently in this Company;    -   Creating different polymerization zones from which the stress        developed in a polymerized zone could be transferred to its        adjacent unpolymerized zone and got relief like segmental        polymerization technique developed in this company (U.S. Pat.        No. 6,783,810);    -   Employing different polymerization groups such as hybrid monomer        with (meth)acrylate and vinyl ether.    -   Using large-size macromonomer to limited its reactivity at the        early stage;-   2. Reduce the conversion;    -   Pre-building a 2D or 3D structure like polymerizable        macrocyclics developed in this Company, or dendremers or        hyperbranches-   3. Further tuning radical polymerization kinetic so as to allow    stress relief during the course of cross-linking formation.

The present invention relates to a dental composite composition based ona novel tertiary aryl amine, more specifically a multiple, tertiary arylamine which allow a better control over photopolymerization kinetics soas to generate much less curing contraction stress within a normalcross-linked system. In addition, composition from such novel amine canalso offer improved color stability as comparison to that based on aconventional mono-tertiary aryl amine such as EDBA. Obviously thebenefit from such a concept would not limit in dental composite or otherdental materials. The application can be found in optical coating,microelectronic, et al.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the molecular structure of TMFBP.

FIG. 2 is a table showing the thermal aging effect on color change forcured TPH resin systems with EDAB or TMFBP at 50° C. in air.

FIG. 3 is a table showing color stability for thick, cured TPH resinwith EDAB or TMFBP at 25° C. in air.

SUMMARY OF THE INVENTION

The present invention is related to the use of a tertiary aromaticdiamines as coinitiators or accelerator with conventional photoinitiatorin light curable resin systems and/or any other resin compositions toreduce the discoloration of aromatic diamines in the said systems and toreduce the shrinkage stress from photopolymerization as well. Moreparticularly, this invention relates to a class of tertiary aromaticdiamines having twisted biphenyl moiety which not only demonstrateimproved color stability of the resulting light curable resin and paste,but also offer reduced polymerization stress of the same resin and/orpaste therefrom, which is highly desirable especially for dentalrestoratives.

Novel tertiary aromatic diamines containing twisted biphenyl moiety wasinitially designed for improved color stability for its usage asaccelerator with alfa-diketone photoinitiator, such as camphorquinone.We have discovered that such tertiary aromatic diamines are veryeffective in improving color stability. Unlike prior tertiary aromaticamine used in dental formulation, such as EDAB, the tertiary aromaticamines of this invention contain twisted biphenyl moiety and multipleamine groups. Because of these multiple amine groups and its uniquestructure feature, the diamines of this invention are expected moresusceptive to light-induced tautomerization or more stable than priortertiary amines, which also results in a different photo-reactivity ascoinitiator.

We have also found that a distinguished photopolymerization kinetics ofthose resins or pastes from such new amine systems is demonstrated.Thus, another unique feature for those resin or paste containing suchtertiary aromatic diamine can effectively reduce contraction stress viaan effective stress dispersion through such extended network settingprocess. Furthermore, significantly improved color stability for thoseresin and/or paste composition which contain such a new amine have beendemonstrated, even with higher amine content.

The monomer to be used in this invention may be any of monomers with atleast one polymerizable olefinic unsaturated group. Among them,(meth)acrylate monomers are preferably used as the monomer. The(meth)acrylate monomer to be used preferably in this invention may beany of mono-functional (meth)acrylate monomers andpolyfunctional(meth)acrylate monomers.

The preferable polymerizable composition for dental use may contain oneor more the (meth)acrylate monomer described above so as to offerultimate balanced overall performance, including mechanical strength,handling characteristics, improved color stability, reducedpolymerization shrinkage and shrinkage stress.

DESCRIPTION OF RELATED ART

Aromatic tertiary amines have been widely used as coinitiator oraccelerator for alpha-diketone-based photoinitiation system, which isusually in an amount of 0.01 to 5.0% by weight based on the amount ofpolymerizable resins. However, residual amine can cause an undesiredcolor change, which would lead a color mismatch. In order to prevent thediscoloration of cured systems, extra UV stabilizer such Uvinol has tobe used in the composition in 0.5-2.0% wt/wt.

The use of photo stabilizers in any polymer matrix is well known in theart. These stabilizers are often used to protect the polymer or coatingfrom the effects of heat, light, and ultraviolet radiation. Primaryantioxidants, such as hindered phenolics, are often used to protectagainst oxidation at elevated temperatures. UV absorbers, such as thebenzotriazoles, and hindered amine light stabilizers can be used toretard discoloration (yellowing) of cured polymers. Thus, it isdesirable to develop a novel amine for replacing current EDAB so as toimprove the color stabilization. Preferably, the new amine would preventthe discoloration of the aromatic diamine.

Amine content is critical in balance of polymerization reactivity andstorage stability. Increasing the concentration of EDAB will lead to theincreasing reactivity, which would improve the mechanical strength andincrease depth of cure, especially for dark shade. However, the downside for this is the increasing ambient light sensitivity and upcomingshade shift due to color instability of residual amine. It was believedthat the oxidation of residual amine promotes a quinonoid formation,leading to color change. The new amine is based on a twisted biphenyldiamine, 2,2′-trifluromethyl-4,4′-biphenyldiamine (TFMB). One-stepmethylation-reaction results in a tertiaryldiamine,N,N,N′,N′-tetramethyl-2,2′-trifluromethyl-4,4′-biphenyldiamine (TMFBP),see FIG. 1. It is expected such twisted amine would not only offerbetter solubility in resin system, but also demonstrate good colorstability. On the other hand, it is also expected an unique reactionkinetics for such amine/resin system because of the featured diamine.TMFBP can be cored out for chain radiation growth unlike the one-waychain growth with a dead-end by any monoamine, such as EDAB.

As showed in Table I and Table II, new amine, TMFBP, was formulated withCQ/TPH resin and EDAB as control. Standard Harpoon filler formulationwas used to make the composite pastes. Obviously, no significantdifference between EDBA- and TMFBP-based resin and pastes was found forsame amine level. However, the weak property from paste based on lowlevel EDAB, LB5-188, suggested lower reactivity for that system ascompared TMFBP system. More importantly, the accelerated aging test onthe cured chips from neat resins, LB5-178-1 and LB5-178-2 vs. LB5-178-3and LB5-178-4, showed that larger color shit (ΔE) was found forLB5-178-2 than LB5-178-4, 1.14 vs. 0.59. Extended aging test is inprocess. In addition, it was also noted that for same level CQ/EDAB andfiller loading, TPH resin-based paste, LB5-188 still offer much highershrinkage stress (2.6 MPa vs. 1.5 MPa) than those P&P-resin-basedcomposite pastes as reported previously. The effect of new amine in P&Presin system will be investigated next.

To find out the last standing level for TMFBP in CQ/TPH resin system, arange of CQ/TMFBP composition was examined as showed in Table III andpastes were prepared accordingly (Table IV). Although there is nosignificant difference in polymerization shrinkage among these resinformulations, LB5-192-1, -2, -3, -4, -5, and -6, difference in shrinkagestress was revealed. Furthermore, there appears a critical amine levelthat governs the total shrinkage stress (FIG. 2). Similar effect of CQon shrinkage stress was also noted, see FIG. 3. However, thesignificance of amine level on the shrinkage stress for thecorresponding composite pastes was dimmed as showed in Table IV.Additional investigation is needed to identify the minimum TMFBP levelin TPH resin system and to further evaluate the paste's color stability.A patent will be filled in this regard.

The invention will be further described in connection with the followingexamples that are set forth for purpose of illustration only.

EXAMPLE 1

Preparation of Tertiary Twisted Biphenyldiamine

As illustrated above, typical 2,2′-substituted biphenyl-N,N′-dimethyldiamine can be prepared directly from corresponding twisteddiamines according to a procedure described in “Organic Synthesis, Coll.Vol. V, p 1084-87” for a mono-tertiarydiamine, m-Trifluoromethyl-N,N-dimethylamine. Similarly 2,2′,6,6′-substitutedbiphenyl-N,N′-dimethyldiamine (see below) can prepared as well. However,other methods to prepare the twisted tertiary diamine with same orsimilar structure for same purpose in use will fall into the scope ofthis invention.

Add 28.6 grams (0.102 mole) of trimethyl phosphate into a 500 mlround-bottomed three-neck flask with a mechanical agitator, N2 inlet anda condenser. Then 21.4 grams (0.05 mole) of 2,2′-bis(trifluoromethyl)-biphenyl-4, 4′-diamine (TFMB) was added into thesystem. Clear solution was soon developed and refluxed. The stirredreaction mixture was gradually heated by an oil bath to approximately150 over 1-2 hrs, then the temperature has to be up to 200° C. so as toavoid the sublimation of the reactant in the flask. After 2-3 hrs, thereaction mixture is cooled to room temperature. Then a solution of 30grams of NaOH in 200 ml of water is added to the flask with solidifiedreaction mixture and the mixture was carefully and vigorously stirredfor 2 hrs. Insoluble crystalline solid is collected after filtration.Then it is recrystallized with acetone a couple of time. The structureof the resulting product is confirmed by NMR spectroscopy analysis (1H,F16 and C13).

EXAMPLE 2 Activated Resin Formulation with TFMBP

An activated resin mixture is prepared by mixing 30.0 grams of TPHresin, a mixture of EBPADMA and TEGDMA, 0.15 wt/wt % of CQ, 0.20 wt/wt %of TFMBP and 0.02 wt/wt % of BHT at 50.0° C. for 1 hr. Such an activatedresin was thoroughly evaluated by polymerization shrinkage, shrinkagestress, conversion, and polymerization rate, which are summarized inTable I.

EXAMPLE 3 Light Curable Composite Containing TFMBP

The activated resin mixture as made above was then formulated into acomposite paste with loading up to 82 wt/wt % of inorganic glass fillersas described elsewhere. Such paste was evaluated thoroughly as listed intable II.

COMPARABLE EXAMPLE 1 Activated Resin Formulation with EDAB

An activated resin mixture is prepared by mixing 30.0 grams of TPHresin, a mixture of EBPADMA and TEGDMA, 0.15 wt/wt % of CQ, 0.20 wt/wt %of EDAB and 0.02 wt/wt % of BHT at 50.0° C. for 1 hr. Such an activatedresin was thoroughly evaluated by polymerization shrinkage, shrinkagestress, conversion, and polymerization rate, which are summarized inTable Ia and Ib.

EXAMPLE 2 Light Curable Composite Containing EDAB

It was then formulated into a composite paste with loading up to 82wt/wt % of inorganic glass fillers as described elsewhere. Such pastewas evaluated thoroughly as listed in table IIa and IIb.

TABLE Ia Composition and Properties for Activated Resins with DifferentAmines 70% P&P Resin 70% P&P Resin 15% TPH Resin 15% TPH Resin 85% P&PResin 85% P&P Resin 15% Diluent 147 15% Diluent 147 15% TEGDMA 15%TEGDMA 0.15% CQ 0.15% CQ 0.15% CQ 0.15% CQ 0.20% EDAB 0.20% TMFBP 0.20%EDAB 0.20% TMFBP Lot # LB6-9-5 LB6-9-2 LB6-15-2 LB6-15-1 Viscosity@20°C. 410 510 245 270 poise Uncured density 1.1088 1.1087 1.1225 1.1231g/cm³ Cured density 1.1830 1.1826 1.2074 1.2086 g/cm³ Shrinkage @ 24 hrs% 6.27 6.25 7.03 7.07 Stress @ 60 min 1.43 0.96 2.05 1.58 MPa ΔH₁ in N2113 111 137 134 w/o UV filter t_(o) 16.2 17.4 10.8 12.0 seconds t_(max)37.2 49.2 35.4 38.4 seconds ΔH₁ in N2 115 109 134 130 w/ UV filter t_(o)18.6 19.8 11.4 13.2 seconds t_(max) 45.0 53.4 35.4 43.8 seconds

TABLE Ib Composition and Properties for Activated Resins with DifferentAmines 100% P&P Resin 100% P&P Resin 0.15% CQ 0.15% CQ 0.20% EDAB 0.20%TMFBP 0.02% BHT 0.02% BHT Lot # LB6-23-1 LB6-23-2 Viscosity@20° C. 15501550 poise Uncured density 1.1319 1.1326 g/cm³ Cured density 1.20561.2059 g/cm³ Shrinkage @ 24 hrs % 6.11 6.08 Stress @ 60 min 1.55 1.17MPa ΔH₁ in N2 110 103 w/o UV filter t_(o) 22.2 25.2 seconds t_(max) 46.862.4 seconds ΔH₁ in N2 100 102 w/ UV filter t_(o) 22.8 35.4 secondst_(max) 52.2 70.2 seconds

TABLE Ic Amine Effect on Polymerization Stress for Comparable ResinsResins w/ EDAB w/ TMFBP Stress Reduction, % LB6-9-5 1.4 LB6-9-2 1.0 −29LB6-15-2 2.1 LB6-15-1 1.6 −24 LB6-23-1 1.6 LB6-23-2 1.2 −25

TABLE Id Effect of Initiator Composition on Polymerization Shrinkage andStress for Activated TPH Resin TPH Stress Resin CQ %, TMFBP Shrinkage @60 min Lot # wt/wt %, wt/wt %, wt/wt @ 24 hrs % MPa LB5-192-1 100 0.100.10 7.14 3.0 LB5-192-2 100 0.10 0.15 7.17 3.4 LB5-192-3 100 0.10 0.207.11 3.7 LB5-192-6 100 0.20 0.10 7.10 4.0 LB5-192-4 100 0.20 0.20 7.224.2 LB5-192-5 100 0.25 0.20 7.16 4.5

TABLE IIa Composition and Properties for Composite with Different AminesPastes LB6-14 LB6-11 LB6-17 LB6-16 Resins LB6-9-5 LB6-9-2 LB6-15-2LB6-15-1 (composition) 18% 18% 17% 17% Filler 82% 82% 83% 83%(Composition) Uncured density 2.1469 2.1719 1.1852 1.1858 g/cm³ Cureddensity 2.1752 2.1840 1.2205 1.2189 g/cm³ Shrinkage @ 24 1.30 0.55 1.591.49 hrs % Stress @ 60 min 1.34 N/A 1.70 1.50 MPa ΔH₁ in N2 21.8 19.821.0 19.6 w/ UV filter t_(o) 12.0 12.0 15.6 18.6 seconds t_(max) 44.449.2 52.8 65.4 seconds Flexural Strength 120 N/A 150 145 MPa FlexuralModulus 8300 N/A 10800 10300 MPa Compressive 330 320 350 340 StrengthMPa Compressive 7600 7600 8000 7800 Modulus MPa

TABLE IIb Composition and Properties for Composite with Different AminesPastes LB6-24 LB6-25 Resins LB6-23-1 LB6-23-2 (Composition) 18% 18%Filler 82% 82% (Composition) Uncured density 2.1661 2.1660 g/cm³ Cureddensity 2.1951 2.1926 g/cm³ Shrinkage @ 24 hrs % 1.32 1.21 Stress @ 60min 1.34 0.91 MPa ΔH₁ in N2 16.8 15.5 w/ UV filter t_(o) 23.4 25.2seconds t_(max) 75.0 107.4 seconds Flexural Strength 138 137 MPaFlexural Modulus 11200 10000 MPa Compressive Strength 310 300 MPaCompressive Modulus 8000 8000 MPa

TABLE IIc Amine Effect on Polymerization Stress for Comparable PastesPastes EDAB TMFBP Stress Reduction, % LB6-14 1.3 LB6-11 na na LB6-17 1.7LB6-16 1.5 −12 LB6-24 1.3 LB6-25 0.9 −30

In some embodiments, the tertiary twisted biphenyldiamine has thefollowing generic formula:

where at least one of R and R1 independently of one another or R and R1together of the same to each other are an O-, S- or N containing 5- or6-membered heterocyclic ring; and where each R2 and each R3independently of one another or together of the same to each other isselected from the group consisting of: alkyl having from C1 to C18carbon atoms; and CH₂R5, where R5 is selected from the group consistingof: C1-C2 alkyl or fluorinated alkyl; C2-C8 alkyl or fluorinated alkylinterrupted by one or more O atoms; phenyl-substituted C1-C4 alkyl;C2-C18 alkenyl; phenyl which is unsubstituted or is substituted from oneto five times by halogen, hydroxyl, C1-C8 alkyl and/or C1-C8 alkoxyl;naphthyl which is unsubstituted or substituted one to five times byhalogen, hydroxyl, C1-C8 alkyl and/or C1-C8 alkoxyl; biphenyl which isunsubstituted or substituted from one to five times by halogen,hydroxyl, C1-C8 alkyl and/or C1-C8 alkoxyl, C3-C12 cycloalkyl; and an O-or N-containing 5- or 6-membered heterocyclic ring.

Thus, it should be evident that the invention as disclosed hereincarries out one or more of the objects of the present invention setforth above and otherwise constitutes an advantageous contribution tothe art. As will be apparent to persons skilled in the art,modifications can be made to the preferred embodiments disclosed hereinwithout departing from the spirit of the invention, the scope of theinvention herein being limited solely by the scope of the attachedclaims.

What is claimed is:
 1. A photopolymerizable composition comprising: a2-substituted biphenyl tertiary diamine having a twisted biphenylmoiety, a photosensitizer, and at least one curable resin; wherein theat least one curable resin comprises at least one of a monofunctional(meth)acrylate or a polyfunctional (meth)acrylate; and wherein the2-substituted biphenyl tertiary diamine having the twisted biphenylmoiety has the structure:

wherein at least one of R and R1 independently of one another or R andR1 together of the same to each other are an O-, S- or N containing 5-or 6-membered heterocyclic ring; and wherein each R2 and each R3independently of one another or together of the same to each other isselected from the group consisting of: alkyl having from C1 to C18carbon atoms; and CH₂R5, wherein R5 is selected from the groupconsisting of: C1-C2 alkyl or fluorinated alkyl; C2-C8 alkyl orfluorinated alkyl interrupted by one or more O atoms; phenyl-substitutedC1-C4 alkyl; C2-C18 alkenyl; phenyl which is unsubstituted or issubstituted from one to five times by halogen, hydroxyl, C1-C8 alkyland/or C1-C8 alkoxyl; naphthyl which is unsubstituted or substituted oneto five times by halogen, hydroxyl, C1-C8 alkyl and/or C1-C8 alkoxyl;biphenyl which is unsubstituted or substituted from one to five times byhalogen, hydroxyl, C1-C8 alkyl and/or C1-C8 alkoxyl, C3-C12 cycloalkyl;and an O- or N-containing 5- or 6-membered heterocyclic ring.
 2. Acomposition as in claim 1, wherein at least one of R and R1 is anelectron-withdrawing group.
 3. A composition as in claim 2, wherein the2-substituted biphenyl tertiary diamine having the twisted biphenylmoiety is present in the amount of 0.05-1.5% wt/wt.
 4. A composition asin claim 1, wherein the O-, S- or N containing 5- or 6-memberedheterocyclic ring is selected from the group consisting of furyl,pyrrolyl, dioxinyl, and pyridyl.
 5. A composition as in claim 1, whereineach R2 and each R3 independently of one another or together of the sameto each other is an alkyl having from 1 to 5 carbon atoms.
 6. Acomposition as in claim 1, wherein alkyl each R2 and each R3independently of one another or together of the same to each other is analkyl having from 1 to 18 carbon atoms.